In order to increase the sensitivity and decrease the size of charge-coupled device (CCD) image sensors, the individual photosites that the arrays are designed with are made to be smaller. This smaller size allows for more photosites within the image area (i.e. higher density, higher resolution) and consequently a smaller amount of semiconductor substrate is required for any particular size array.
A necessary consequence of this reduction in scale is the fact that during the device's integration time, less charge will be accumulated in any individual photosite due to the fact that its photon collection area comprises a smaller portion of the entire image array.
Essential to good low noise performance of a CCD image sensor is the charge detection amplifier which converts the charge stored at an individual photosite into a signal of adequate magnitude for further processing. Typical charge conversion sensitivity in conventionally available sensors ranges from approximately 4.0 to 10.0 microvolts per electron. This performance level is unsatisfactory for image sensors having high density photosite arrays in which the charge accumulated in each photosite may be only a few tens of electrons.
Referring to FIG. 1, a prior art charge detection amplifier is shown. The circuit consists of a detection node 40 (typically a reverse biased n-p junction), a reset element Q1, reference voltage terminal 48, and dual stage source follower 50 consisting of transistors Q2 and Q3 and constant current sources 44 and 46. Capacitor 42 represents the total parasitic capacitance and includes the diode capacitance of the CCD detection node as well as the input capacitance of the source follower 50 (C.sub.GD, C.sub.GS, etc.). Even though this circuit has been used widely and is simple, it has the disadvantages of no voltage gain (source follower A.sub.v is typically around 0.7), low charge conversion factor (due to the parasitic capacitance C.sub.p), and large kTC noise due to the reset requirement of the detection node and large parasitic capacitance C.sub.p. To minimize the undesirable effects of the large kTC noise necessitates complex signal processing, a clearly undesirable consequence.